Improved aqueous fracturing fluids have now been discovered that are particularly useful as well stimulation fluids to fracture tight (i.e., low permeability) subterranean formations. Gas wells treated with these fracturing fluids have rapid cleanup and enhanced well production. The fluids contain small but sufficient amounts of certain amine oxides to aid in the removal of the fracturing fluid from the formation. By facilitating the removal of fluid from the invaded zones, the amount of damage to the fracture faces in the formation is thereby minimized.
Various amine oxides have been used as surfactants to create foams and remove xe2x80x9cintrusion fluids from wellbores,xe2x80x9d according to U.S. Pat. No. 3,303,896 and they have been used as foam stabilizers, according to U.S. Pat. No. 3,317,430. Certain amine oxides have also been used in combination with quaternary ammonium compounds as foaming and silt suspending agents. See, for example, U.S. Pat. No. 4,108,782 and U.S. Pat. No. 4,113,631. The use of amine oxide surfactants for chemical flooding enhanced oil recovery was described in a topical report by David K. Olsen in NIPER-417 (August 1989) for work performed for the US Department of Energy under cooperative agreement DE-FC22-83FE60149 by the National Institute for Petroleum and Energy Research. However, to Applicants"" knowledge, the amine oxides have not been used to improve the properties of fracturing fluids and to promote rapid cleanup, or to enhance well production from a well stimulated by hydraulic fracturing.
Hydraulic fracturing of subterranean formations has long been established as an effective means to stimulate the production of hydrocarbon fluids from a wellbore. In hydraulic fracturing, a well stimulation fluid (generally referred to as a fracturing fluid or a xe2x80x9cfrac fluidxe2x80x9d) is injected into and through a wellbore and against the surface of a subterranean formation penetrated by the wellbore at a pressure at least sufficient to create a fracture in the formation. Usually a xe2x80x9cpad fluidxe2x80x9d is injected first to create the fracture and then a fracturing fluid, often bearing granular propping agents, is injected at a pressure and rate sufficient to extend the fracture from the wellbore deeper into the formation. If a proppant is employed, the goal is generally to create a proppant filled zone (aka, the proppant pack) from the tip of the fracture back to the wellbore. In any event, the hydraulically induced fracture is more permeable than the formation and it acts as a pathway or conduit for the hydrocarbon fluids in the formation to flow to the wellbore and then to the surface where they are collected. The methods of fracturing are well known and they may be varied to meet the user""s needs, but most follow this general procedure (which is greatly overly simplified).
The fluids used as fracturing fluids have also been varied, but many if not most are aqueous based fluids that have been xe2x80x9cviscosifiedxe2x80x9d or thickened by the addition of a natural or synthetic polymer (cross-linked or uncross-linked). The carrier fluid is usually water or a brine (e.g., dilute aqueous solutions of sodium chloride and/or potassium chloride). The viscosifying polymer is typically a solvatable (or hydratable) polysaccharide, such as a galactomannan gum, a glycomannan gum, or a cellulose derivative. Examples of such polymers include guar, hydroxypropyl guar, carboxymethyl guar, carboxymethylhydroxyethyl guar, hydroxyethyl cellulose, carboxymethyl-hydroxyethyl cellulose, hydroxypropyl cellulose, xanthan, polyacrylamides and other synthetic polymers. Of these, guar, hydroxypropyl guar and carboxymethlyhydroxyethyl guar are typically preferred because of commercial availability and cost performance.
In many instances, if not most, the viscosifying polymer is crosslinked with a suitable crosslinking agent. The crosslinked polymer has an even higher viscosity and is even more effective at carrying proppant into the fractured formation. The borate ion has been used extensively as a crosslinking agent, typically in high pH fluids, for guar, guar derivatives and other galactomannans. See, for example, U.S. Pat. No. 3,059,909 and numerous other patents that describe this classic aqueous gel as a fracture fluid. Other crosslinking agents include, for example, titanium crosslinkers (U.S. Pat. No. 3,888,312), chromium, iron, aluminum, and zirconium (U.S. Pat. No. 3,301,723). Of these, the titanium and zirconium crosslinking agents are typically preferred. Examples of commonly used zirconium crosslinking agents include zirconium triethanolamine complexes, zirconium acetylacetonate, zirconium lactate, zirconium carbonate, and chelants of organic alphahydroxycorboxylic acid and zirconium. Examples of commonly used titanium crosslinking agents include titanium triethanolamine complexes, titanium acetylacetonate, titanium lactate, and chelants of organic alphahydroxycorboxylic acid and titanium.
Additional information on fracturing is found in the description by Janet Gulbis and Richard M. Hodge in Chapter 7 of the text xe2x80x9cReservoir Stimulationxe2x80x9d published by John Wiley and Sons, Ltd, Third Edition, 2000 (Editors, Michael J. Economides and Kenneth G. Nolte), which is incorporated herein by reference. Some fracturing fluids have also been energized by the addition of a gas (e.g., nitrogen or carbon dioxide) to create a foam. See, for example, the pioneering work by Roland E. Blauer and Clarence J. Durborow in U.S. Pat. No. 3,937,283 (xe2x80x9cFormation Fracturing with Stable Foamxe2x80x9d). The rheology of the traditional water-base polymer solutions and also complex fluids, such as foams, can be and typically is modified and augmented by several additives to control their performance. Fluid loss additives are typically added to reduce the loss of fracturing fluids into the formation.
The problems associated with the loss of fracturing fluid to the formation are well known. For example, in 1978 Holditch reported: xe2x80x9cThe fluid injected during the fracturing treatment will leak off into the formation and will reduce the relative permeability to gas in the invaded region. Near the fracture, the permeability to gas will be reduced to zero.xe2x80x9d In addition, Holditch said: xe2x80x9cIn some cases, the injected fracturing fluid may reduce the formation permeability in the invaded zone.xe2x80x9d Stephen A. Holditch, SPE 7561 (Presented at the 53rd Annual Fall Technical Conference and Exhibition of the Society of Petroleum Engineers of AIME, held in Houston, Tex., Oct. 1-3, 1978). The damage to the formation could be severe, and the practical so what of that is reduced flow of hydrocarbons, low production and poor economics on the well. While the state of the art has advanced substantially since Holditch reported on the problems associated with leak off of fracturing fluid, the problems remain the same. See, for example, Vernon G. Constien, George W. Hawkins, R. K. Prud""homme and Reinaldo Navarrete, Chapter 8 entitled xe2x80x9cPerformance of Fracturing Materialsxe2x80x9d and the other chapters on fracturing and well stimulation in xe2x80x9cReservoir Stimulationxe2x80x9d published by John Wiley and Sons, Ltd, Third Edition, copyright Schlumberger 2000 (Editors, Michael J. Economides and Kenneth G. Nolte), the disclosure of which is incorporated herein by reference. These authors and others emphasize the importance of xe2x80x9ccleanupxe2x80x9d or xe2x80x9cfracture cleanupxe2x80x9d to optimize production of the hydrocarbon fluids from the well. The term xe2x80x9ccleanupxe2x80x9d or xe2x80x9cfracture cleanupxe2x80x9d refers to the process of removing the fracture fluid (without the proppant) from the fracture after the fracturing process has been completed. Techniques for promoting fracture cleanup often involved reducing the viscosity of the fracture fluid as much as practical so that it will more readily flow back toward the wellbore. So-called xe2x80x9cbreakersxe2x80x9d have been used to reduce fluid viscosity in many instances. The breakers can be enzymes (oxidizers and oxidizer catalysts), and they may be encapsulated to delay their release. See, for example, U.S. Pat. No. 4,741,401 (Walles et al.), assigned to Schlumberger Dowell and incorporated herein by reference. Another technique to aid in the cleanup, albeit by a contrarian approach, is found in U.S. Pat. No. 6,283,212 (Hinkel and England), which is also assigned to Schlumberger Dowell and incorporated herein by reference.
The need for improved fracturing fluids still exists, and the need is met at least in part by the following invention.
Improved aqueous fracturing fluids have now been discovered that are particularly useful as well stimulation fluids to fracture tight (i.e., low permeability) subterranean formations. Gas wells treated with these fracturing fluids have rapid cleanup and enhanced well production. The fluids contain small but sufficient amounts of certain amine oxides to aid in the removal of the fracturing fluid from the formation. By facilitating the removal of fluid from the invaded zones, the amount of damage to the fracture faces in the formation is thereby minimized. The amine oxides correspond to the formula 
wherein R1 is an aliphatic group of from 6 to about 20 carbon atoms, and wherein R2 and R3 are each independently alkyl of from 1 to about 4 carbon atoms. The amine oxides in which R1 is an alkyl group are preferred, and those in which R1 is an alkyl group of from 8 to 12 carbon atoms (in particular where R1 is a linear alkyl group), and R2 and R3 are each methyl or ethyl groups are most preferred.